Acid rain refers to any form of precipitation (rain, snow, fog, dew) whose pH falls below the natural value of 5.6 due to the presence of dissolved acids in the atmosphere. The main culprits are sulphur dioxide (SO₂) and nitrogen oxides (NOₓ), emitted from burning fossil fuels, which react with water vapour to form sulphuric acid and nitric acid.
Formation and transport
Precursor gases can travel hundreds or thousands of kilometres before being deposited, so acid rain can affect regions far removed from emission sources. Deposition can be wet (rain, snow) or dry (particles and gases depositing directly on surfaces without precipitation).
Its effects are devastating: acidification of lakes and rivers (eliminating fish and amphibians), forest damage (leaf weakening, root death), deterioration of limestone and marble buildings and monuments, and soil depletion. In Europe, acid rain caused severe damage to forests in Scandinavia and Central Europe during the 1970s-80s.
Environmental regulations (flue-gas desulphurisation, vehicle catalytic converters) have drastically reduced SO₂ emissions in Europe and North America, improving the situation. In Spain, SO₂ emissions have been reduced by 90 % since 1990. However, the problem persists in industrialised regions of Asia. See also: climate change.